The use of thermoplastic stretch wrap for the overwrap packaging of goods, and in particular, the unitizing of pallet loads, is currently developing commercial end use application for thermoplastic films including, generically, polyethylene. There are a variety of overwrapping techniques which are employed utilizing such stretch wrap films, including locating the pallet load to be wrapped atop a rotating platform. As the stretch wrap film is laid on about the girth of the pallet load, the pallet load is rotated on its platform. The stretch wrap is applied from a continuous roll thereof. In one type of wrapper, braking tension is applied to the continuous roll of film so that the film is being continuously stretched by the rotating pallet load. Usually the stretch wrap film, located adjacent to the rotating pallet load, is vertically positioned and the rotating platform or turntable may be operated at speeds ranging from about 5 to about 50 revolutions per minute. At the completion of the overwrap operation, the turntable is stopped completely while the film is cut and attached to the previous layer of film by employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overlapped may be shrouded in the film while the vertically positioned film roll remains fixed in a vertical position, or the vertically positioned film roll (for example in the case of relatively narrow film widths and relatively wider pallet loads) may be arranged to move in a vertical direction as the load is being overwapped whereby a spiral wrapping effect is achieved on the package goods.
Some of the properties desired of a good stretch wrap film are as follows:
Good cling or cohesion properties. PA0 Good transparency. PA0 Low stress relaxation with time. PA0 Good puncture resistance. PA0 High resistance to transverse tear when under machine direction tension. PA0 Producible in thin gauges. PA0 Low specific gravity and thus high yield in area per pound. PA0 High machine direction ultimate tensile strength. PA0 High machine direction ultimate elongation. PA0 High modulus of elasticity. PA0 High tear resistance in the transverse direction. PA0 High puncture resistance.
The recently developed linear low density polyethylenes have the desired strength and toughness properties required from stretch wrap applications, but do not possess the required cling, which in the case of conventional polyethylene, can be partially obtained by incorporation of vinyl acetate as a comonomer.
It is known in the art to include cling additives in order to impart an increased cling force between two contacting films. For example, household cling wrap film, used to cover dishes containing leftover food must have the ability to cling to smooth surfaces made of glass, ceramic, and plastic. Examples of such cling wrap film are described in U.S. Pat. Nos. 4,348,455 and 4,367,256 the disclosures of which are in their entirety incorporated herein by reference. In both of these teachings the cling wrap additive (alkali metal dialkyl sulfosuccinate in one case, and glycerol oleates in the other case) are incorporated uniformly throughout the film so that the cling force on one side of the film is substantially the same as that of the other side of the film.
The incorporation of such cling agents in a stretch wrap film employed for the unitizing of pallet loads presents a serious and expensive problem. While the employment of such a cling modified stretch wrap film will enhance and strengthen the bonding of the overlap film after the load has been stabilized there if the tendency of one stabilized load to cling or drag against an adjacent load causing transportation and handling problems. Because of the cling nature of the overwrap, one such overwrapped pallet load will not slide against an adjacent load. The tendency is for one load to pull destructively upon an adjacent load because of the clinging nature of the film. Pallet loads are moved about by fork lift trucks and the forces involved can and do pull through the stretch wrap film and destroy the integrity of the overwrapped load. An example of a film possessing cling properties on one side, while the other side exhibits substantially no cling force when in contact with a layer of itself, is provided in U.S. Pat. No. 4,518,654, the disclosure of which is hereby incorporated by reference in its entirety.
U.S. Pat. No. 4,518,654 discloses a bi-layer A/B film structure wherein the base polymer of layer A, possessing no substantial cling characteristics of its own, is modified by the uniform dispersion therein of a cling additive in an amount sufficient to maintain layer A in a stretched condition in cling contact with layer B. Layer B contains an anticling additive, uniformly dispersed, of a character and in an amount sufficient to prevent any substantial cling force to be exerted when in contact with a layer of itself. The films disclosed therein are preferably coextruded.
While cling additives, also known as tackifiers, are commonly believed to work by migrating to the surface of a film, their ability to migrate can pose special problems when seeking to provide effective stretch wrap films. For example, should the cling additive migrate to the noncling region of the film and seek residence on the surface of same, the noncling properties of that side of the film will be substantially impaired. Likewise, should the anticling additive migrate to the cling surface of the film, the ability of the cling surface to maintain cling-contact in use may be reduced or destroyed.
In view of the problems associated with additive migration in one-sided cling stretch wraps, it is an object of this invention to provide a polymeric film having the proper combination of properties of overcome these problems and thereby provide stretch wrap films especially well suited for use in palletizing applications.
It is a further object to overcome these problems in an effective and economical manner.
It is a yet further object to provide an improved one-sided cling, one-sided noncling or slip film wherein the noncling side exhibits a low dynamic coefficient of friction, noncling surface to noncling surface.